1. Field of the Invention
This invention pertains generally to millimeter and sub-millimeter wave imaging, and more particularly to a digital regenerative receiver (DRR) configured for outputting a digital signal in response to received pixel amplitude, a multi-band inter-modulated regenerative receiver (IRR), and an antenna-less super regenerative receiver (ASRR).
2. Description of Related Art
Millimeter wave scanners and imagers have applicability for various imaging purposes, such as for detecting concealed objects, loss prevention, screening, quality assurance and security. One primary advantage of these millimeter wave systems is that many common materials are translucent in extremely high frequency (EHF) (e.g., millimeter wave) radio frequency bands. This frequency range is just below the related sub-millimeter range of Terahertz radiation (“T-ray”) range.
In previous receiver designs, Schottky barrier diodes, bolometers, Quantum Cascade Lasers (QCLs) and superconductor techniques have been utilized to construct signal receivers in the millimeter and sub-millimeter bands. Analog regenerative receivers have also been utilized in much lower frequency bands with analog quench circuitry along with analog output filtering.
In order for imagers based on Terahertz and millimeter-waves to become cost effective in a number of application areas, the core pixel circuits within the imaging array need to meet challenging constraints that originate from the system level design and the need for constructing large array structures on-chip. Perhaps one of the more critical constraints is that each pixel must operate at a very low power consumption. This is necessary because when integrated within an array, the total power consumption is multiplied by the number of elements in the array. For example in a square array, the power consumption inflates to n2 for an array of n pixels in width and n pixels in height.
Another major constraint is the required circuit area covered by each pixel. This area constraint is important because a cost-effective pixel array should ideally fit on a wafer, or portion thereof, to facilitate monolithic fabrication and avoid complicated mechanical assembly of multiple array sections.
Another constraint similar to that experienced in CMOS image sensor arrays is the challenge of routing large numbers of analog signals between each pixel in the array and a sampling analog-to-digital converter (ADC).
Implementation of high-resolution systems (i.e., above 100×100 pixels) in the millimeter-wave spectrum, provides a significant challenge toward simultaneously meeting each of these constraints. For example, traditional multiple-stage or heterodyne-based imaging receivers, require supplying a large number of bias currents which lead to increased power dissipation, while the need for a large number of passive devices require prohibitively large silicon area in the context of imaging array structures.
Accordingly, a need exists for high resolution imaging systems operating in the millimeter and sub-millimeter regimes which provide compact and power efficient operation.